Inventory is money sitting on a shelf
If you buy or make physical products to sell, your inventory is one of the largest numbers on your balance sheet, and the way you account for it ripples straight into your profit. Here's the part that surprises owners: buying inventory is not an expense. When you pay $5,000 for goods to resell, you haven't spent money in the profit sense; you've swapped one asset (cash) for another asset (inventory). The cost only becomes an expense — cost of goods sold — at the moment you sell the item.
That single rule is the heart of inventory accounting, and getting it right is what keeps your P&L honest. This guide explains how inventory flows into COGS, and the two costing methods most small businesses actually use: FIFO and weighted average. (General education, not tax or accounting advice — your situation may differ.)
The flow: from purchase to COGS
Picture the journey of one unit:
- You buy it. Cash (asset) goes down; inventory (asset) goes up. No expense, no effect on profit yet.
- It sits in inventory. It's value you own, counted in current assets, and it counts toward your current ratio.
- You sell it. Now its cost moves out of inventory and onto the P&L as COGS, matched against the revenue from that sale. Profit on that sale = price − that unit's cost.
The whole question of "inventory accounting" is really one question: when a unit sells, which cost do you assign to it? If every unit you ever bought cost exactly the same, there'd be no debate. But prices change — the batch you bought in January cost less than the batch in June — so you need a consistent rule for which cost flows out when something sells.
Why the method changes your profit
This isn't academic. Say you bought inventory in two batches:
- 100 units in January at $10 each = $1,000
- 100 units in June at $14 each = $1,400
You now hold 200 units that cost you $2,400 total. In July you sell 120 units for $20 each — $2,400 in revenue. What's your COGS? It depends entirely on which costs you assign to those 120 units.
FIFO: first in, first out
FIFO assumes the oldest inventory sells first. So your 120 sold units are the 100 January units ($10 each) plus 20 of the June units ($14 each):
- COGS = (100 × $10) + (20 × $14) = $1,000 + $280 = $1,280
- Gross profit = $2,400 − $1,280 = $1,120
- Inventory still on hand = 80 June units × $14 = $1,120
FIFO usually mirrors how goods physically move (you sell the old stock first, especially anything perishable). In a period of rising prices, FIFO leaves the newer, higher costs sitting in ending inventory and pushes the older, lower costs into COGS, which means a lower COGS and a higher reported profit. Higher profit can mean a higher tax bill, which is the trade-off to keep in mind.
Weighted average: blend every cost
The weighted-average method ignores which specific batch sold and instead averages all your costs into one blended per-unit cost:
- Total cost ÷ total units = $2,400 ÷ 200 = $12 per unit
- COGS on 120 units = 120 × $12 = $1,440
- Gross profit = $2,400 − $1,440 = $960
- Inventory on hand = 80 × $12 = $960
Same sales, same purchases — but FIFO reported $1,120 in profit and weighted average reported $960. Neither is "wrong"; they're different consistent rules. Weighted average is simpler to run, smooths out price swings, and is a natural fit when your units are interchangeable (think screws, fuel, raw material) and tracking individual batches would be busywork.
Periodic vs. perpetual: how often you count
Separately from which costs flow, there's when you measure:
- Periodic — you count inventory at the end of a period (a month-end or year-end physical count) and back into COGS from the change. Simple, cheap, but you're flying blind between counts.
- Perpetual — every sale updates inventory and COGS in real time, so you always know what you hold. This is what inventory-aware software gives you, and it's what makes a mid-month gross-margin number trustworthy.
Most growing product businesses move toward perpetual tracking because guessing your stock level is how you end up either out of stock or drowning in dead inventory.
A few rules that keep inventory honest
- Pick one method and stay with it. Consistency is the point: switching methods to flatter a given year's profit is exactly what auditors and tax rules push back on. Choose deliberately, document it, and keep it.
- Cost means landed cost. A unit's cost isn't just the sticker price; it includes freight-in, duties, and other costs to get it ready to sell. Leaving those out understates COGS and inflates margin — the same discipline as clean expense categorization.
- Write down what you can't sell. Inventory that's damaged, obsolete, or worth less than you paid should be written down to its real value. Carrying dead stock at full cost overstates both your assets and your profit. The mechanics of the entry — and of the shrinkage you find when the count disagrees with the ledger — are covered in inventory shrinkage and write-downs.
- Count it for real. A physical count, reconciled against the books, is to inventory what bank reconciliation is to cash. Shrinkage, miscounts, and theft only show up when the recorded quantity meets the shelf.
The owner's takeaway
If you sell products, inventory accounting isn't optional bookkeeping trivia — it's the machinery that sets your gross margin and your taxable profit. Buying stock isn't an expense; selling it is. Pick FIFO or weighted average on purpose, include every landed cost, write down what won't sell, and count what you hold. Do that and your gross margin reflects the real economics of your business instead of an accident of which batch you happened to assign. Hosting Books tracks inventory as its own asset and moves cost into COGS as items sell, so your margin updates with each sale rather than waiting for a year-end count.
This article is general educational information about accounting concepts and is not tax or accounting advice for your specific situation. Inventory rules and method choices can have tax consequences — confirm with a qualified professional.